Excel Services error: You do not have permissions to open this file

I was configuring Excel Services and got this error

The following steps helped me get rid of it:

1. Open Central Administration -> go to Operations tab
- ensure that the Excel Service is running

2. Open Central Administration -> go to your configured Shared Service -> click Excel Service Settings
- File Access Method: ensure that it is not using Impersonation, instead the Option Process Account should be enabled.

3. Open Central Administration -> go to your configured Shared Service -> click add new trusted file location
- field URL: here you can specify a report library or the whole portal
- Location Type: should be Windows SharePoint Services
- Children trusted: defines whether the children should also be trusted or only the definied path

I had spaces in my doc lib name so I tried adding only the top site name and having its children trusted..it worked for me